
# ABSTRACT: useful, demonstrative, or stupid Sub::Exporter tricks
# PODNAME: Sub::Exporter::Cookbook

#pod =head1 OVERVIEW
#pod
#pod Sub::Exporter is a fairly simple tool, and can be used to achieve some very
#pod simple goals.  Its basic behaviors and their basic application (that is,
#pod "traditional" exporting of routines) are described in
#pod L<Sub::Exporter::Tutorial> and L<Sub::Exporter>.  This document presents
#pod applications that may not be immediately obvious, or that can demonstrate how
#pod certain features can be put to use (for good or evil).
#pod
#pod =head1 THE RECIPES
#pod
#pod =head2 Exporting Methods as Routines
#pod
#pod With Exporter.pm, exporting methods is a non-starter.  Sub::Exporter makes it
#pod simple.  By using the C<curry_method> utility provided in
#pod L<Sub::Exporter::Util>, a method can be exported with the invocant built in.
#pod
#pod   package Object::Strenuous;
#pod
#pod   use Sub::Exporter::Util 'curry_method';
#pod   use Sub::Exporter -setup => {
#pod     exports => [ objection => curry_method('new') ],
#pod   };
#pod
#pod With this configuration, the importing code may contain:
#pod
#pod   my $obj = objection("irrelevant");
#pod
#pod ...and this will be equivalent to:
#pod
#pod   my $obj = Object::Strenuous->new("irrelevant");
#pod
#pod The built-in invocant is determined by the invocant for the C<import> method.
#pod That means that if we were to subclass Object::Strenuous as follows:
#pod
#pod   package Object::Strenuous::Repeated;
#pod   @ISA = 'Object::Strenuous';
#pod
#pod ...then importing C<objection> from the subclass would build-in that subclass.
#pod
#pod Finally, since the invocant can be an object, you can write something like
#pod this:
#pod
#pod   package Cypher;
#pod   use Sub::Exporter::Util 'curry_method';
#pod   use Sub::Exporter -setup => {
#pod     exports => [ encypher => curry_method ],
#pod   };
#pod
#pod with the expectation that C<import> will be called on an instantiated Cypher
#pod object:
#pod
#pod   BEGIN {
#pod     my $cypher = Cypher->new( ... );
#pod     $cypher->import('encypher');
#pod   }
#pod
#pod Now there is a globally-available C<encypher> routine which calls the encypher
#pod method on an otherwise unavailable Cypher object.
#pod
#pod =head2 Exporting Methods as Methods
#pod
#pod While exporting modules usually export subroutines to be called as subroutines,
#pod it's easy to use Sub::Exporter to export subroutines meant to be called as
#pod methods on the importing package or its objects.
#pod
#pod Here's a trivial (and naive) example:
#pod
#pod   package Mixin::DumpObj;
#pod
#pod   use Data::Dumper;
#pod
#pod   use Sub::Exporter -setup => {
#pod     exports => [ qw(dump) ]
#pod   };
#pod
#pod   sub dump {
#pod     my ($self) = @_;
#pod     return Dumper($self);
#pod   }
#pod
#pod When writing your own object class, you can then import C<dump> to be used as a
#pod method, called like so:
#pod
#pod   $object->dump;
#pod
#pod By assuming that the importing class will provide a certain interface, a
#pod method-exporting module can be used as a simple plugin:
#pod
#pod   package Number::Plugin::Upto;
#pod   use Sub::Exporter -setup => {
#pod     into    => 'Number',
#pod     exports => [ qw(upto) ],
#pod     groups  => [ default => [ qw(upto) ] ],
#pod   };
#pod
#pod   sub upto {
#pod     my ($self) = @_;
#pod     return 1 .. abs($self->as_integer);
#pod   }
#pod
#pod The C<into> line in the configuration says that this plugin will export, by
#pod default, into the Number package, not into the C<use>-ing package.  It can be
#pod exported anyway, though, and will work as long as the destination provides an
#pod C<as_integer> method like the one it expects.  To import it to a different
#pod destination, one can just write:
#pod
#pod   use Number::Plugin::Upto { into => 'Quantity' };    
#pod
#pod =head2 Mixing-in Complex External Behavior
#pod
#pod When exporting methods to be used as methods (see above), one very powerful
#pod option is to export methods that are generated routines that maintain an
#pod enclosed reference to the exporting module.  This allows a user to import a
#pod single method which is implemented in terms of a complete, well-structured
#pod package.
#pod
#pod Here is a very small example:
#pod
#pod   package Data::Analyzer;
#pod
#pod   use Sub::Exporter -setup => {
#pod     exports => [ analyze => \'_generate_analyzer' ],
#pod   };
#pod
#pod   sub _generate_analyzer {
#pod     my ($mixin, $name, $arg, $col) = @_;
#pod
#pod     return sub {
#pod       my ($self) = @_;
#pod
#pod       my $values = [ $self->values ];
#pod
#pod       my $analyzer = $mixin->new($values);
#pod       $analyzer->perform_analysis;
#pod       $analyzer->aggregate_results;
#pod
#pod       return $analyzer->summary;
#pod     };
#pod   }
#pod
#pod If imported by any package providing a C<values> method, this plugin will
#pod provide a single C<analyze> method that acts as a simple interface to a more
#pod complex set of behaviors.
#pod
#pod Even more importantly, because the C<$mixin> value will be the invocant on
#pod which the C<import> was actually called, one can subclass C<Data::Analyzer> and
#pod replace only individual pieces of the complex behavior, making it easy to write
#pod complex, subclassable toolkits with simple single points of entry for external
#pod interfaces.
#pod
#pod =head2 Exporting Constants
#pod
#pod While Sub::Exporter isn't in the constant-exporting business, it's easy to
#pod export constants by using one of its sister modules, Package::Generator.
#pod
#pod   package Important::Constants;
#pod  
#pod   use Sub::Exporter -setup => {
#pod     collectors => [ constants => \'_set_constants' ],
#pod   };
#pod  
#pod   sub _set_constants {
#pod     my ($class, $value, $data) = @_;
#pod  
#pod     Package::Generator->assign_symbols(
#pod       $data->{into},
#pod       [
#pod         MEANING_OF_LIFE => \42,
#pod         ONE_TRUE_BASE   => \13,
#pod         FACTORS         => [ 6, 9 ],
#pod       ],
#pod     );
#pod
#pod     return 1;
#pod   }
#pod
#pod Then, someone can write:
#pod
#pod   use Important::Constants 'constants';
#pod   
#pod   print "The factors @FACTORS produce $MEANING_OF_LIFE in $ONE_TRUE_BASE.";
#pod
#pod (The constants must be exported via a collector, because they are effectively
#pod altering the importing class in a way other than installing subroutines.)
#pod  
#pod =head2 Altering the Importer's @ISA
#pod
#pod It's trivial to make a collector that changes the inheritance of an importing
#pod package:
#pod
#pod   use Sub::Exporter -setup => {
#pod     collectors => { -base => \'_make_base' },
#pod   };
#pod
#pod   sub _make_base {
#pod     my ($class, $value, $data) = @_;
#pod
#pod     my $target = $data->{into};
#pod     push @{"$target\::ISA"}, $class;
#pod   }
#pod
#pod Then, the user of your class can write:
#pod
#pod   use Some::Class -base;
#pod
#pod and become a subclass.  This can be quite useful in building, for example, a
#pod module that helps build plugins.  We may want a few utilities imported, but we
#pod also want to inherit behavior from some base plugin class;
#pod
#pod   package Framework::Util;
#pod
#pod   use Sub::Exporter -setup => {
#pod     exports    => [ qw(log global_config) ],
#pod     groups     => [ _plugin => [ qw(log global_config) ]
#pod     collectors => { '-plugin' => \'_become_plugin' },
#pod   };
#pod
#pod   sub _become_plugin {
#pod     my ($class, $value, $data) = @_;
#pod
#pod     my $target = $data->{into};
#pod     push @{"$target\::ISA"}, $class->plugin_base_class;
#pod
#pod     push @{ $data->{import_args} }, '-_plugin';
#pod   }
#pod
#pod Now, you can write a plugin like this:
#pod
#pod   package Framework::Plugin::AirFreshener;
#pod   use Framework::Util -plugin;
#pod
#pod =head2 Eating Exporter.pm's Brain
#pod
#pod You probably shouldn't actually do this in production.  It's offered more as a
#pod demonstration than a suggestion.
#pod
#pod  sub exporter_upgrade {
#pod    my ($pkg) = @_;
#pod    my $new_pkg = "$pkg\::UsingSubExporter";
#pod
#pod    return $new_pkg if $new_pkg->isa($pkg);
#pod
#pod    Sub::Exporter::setup_exporter({
#pod      as      => 'import',
#pod      into    => $new_pkg,
#pod      exports => [ @{"$pkg\::EXPORT_OK"} ],
#pod      groups  => {
#pod        %{"$pkg\::EXPORT_TAG"},
#pod        default => [ @{"$pkg\::EXPORTS"} ],
#pod      },
#pod    });
#pod
#pod    @{"$new_pkg\::ISA"} = $pkg;
#pod    return $new_pkg;
#pod  }
#pod
#pod This routine, given the name of an existing package configured to use
#pod Exporter.pm, returns the name of a new package with a Sub::Exporter-powered
#pod C<import> routine.  This lets you import C<Toolkit::exported_sub> into the
#pod current package with the name C<foo> by writing:
#pod
#pod   BEGIN {
#pod     require Toolkit;
#pod     exporter_upgrade('Toolkit')->import(exported_sub => { -as => 'foo' })
#pod   }
#pod
#pod If you're feeling particularly naughty, this routine could have been declared
#pod in the UNIVERSAL package, meaning you could write:
#pod
#pod   BEGIN {
#pod     require Toolkit;
#pod     Toolkit->exporter_upgrade->import(exported_sub => { -as => 'foo' })
#pod   }
#pod
#pod The new package will have all the same exporter configuration as the original,
#pod but will support export and group renaming, including exporting into scalar
#pod references.  Further, since Sub::Exporter uses C<can> to find the routine being
#pod exported, the new package may be subclassed and some of its exports replaced.
#pod
#pod =cut

__END__

=pod

=encoding UTF-8

=head1 NAME

Sub::Exporter::Cookbook - useful, demonstrative, or stupid Sub::Exporter tricks

=head1 VERSION

version 0.991

=head1 OVERVIEW

Sub::Exporter is a fairly simple tool, and can be used to achieve some very
simple goals.  Its basic behaviors and their basic application (that is,
"traditional" exporting of routines) are described in
L<Sub::Exporter::Tutorial> and L<Sub::Exporter>.  This document presents
applications that may not be immediately obvious, or that can demonstrate how
certain features can be put to use (for good or evil).

=head1 PERL VERSION

This library should run on perls released even a long time ago.  It should
work on any version of perl released in the last five years.

Although it may work on older versions of perl, no guarantee is made that the
minimum required version will not be increased.  The version may be increased
for any reason, and there is no promise that patches will be accepted to
lower the minimum required perl.

=head1 THE RECIPES

=head2 Exporting Methods as Routines

With Exporter.pm, exporting methods is a non-starter.  Sub::Exporter makes it
simple.  By using the C<curry_method> utility provided in
L<Sub::Exporter::Util>, a method can be exported with the invocant built in.

  package Object::Strenuous;

  use Sub::Exporter::Util 'curry_method';
  use Sub::Exporter -setup => {
    exports => [ objection => curry_method('new') ],
  };

With this configuration, the importing code may contain:

  my $obj = objection("irrelevant");

...and this will be equivalent to:

  my $obj = Object::Strenuous->new("irrelevant");

The built-in invocant is determined by the invocant for the C<import> method.
That means that if we were to subclass Object::Strenuous as follows:

  package Object::Strenuous::Repeated;
  @ISA = 'Object::Strenuous';

...then importing C<objection> from the subclass would build-in that subclass.

Finally, since the invocant can be an object, you can write something like
this:

  package Cypher;
  use Sub::Exporter::Util 'curry_method';
  use Sub::Exporter -setup => {
    exports => [ encypher => curry_method ],
  };

with the expectation that C<import> will be called on an instantiated Cypher
object:

  BEGIN {
    my $cypher = Cypher->new( ... );
    $cypher->import('encypher');
  }

Now there is a globally-available C<encypher> routine which calls the encypher
method on an otherwise unavailable Cypher object.

=head2 Exporting Methods as Methods

While exporting modules usually export subroutines to be called as subroutines,
it's easy to use Sub::Exporter to export subroutines meant to be called as
methods on the importing package or its objects.

Here's a trivial (and naive) example:

  package Mixin::DumpObj;

  use Data::Dumper;

  use Sub::Exporter -setup => {
    exports => [ qw(dump) ]
  };

  sub dump {
    my ($self) = @_;
    return Dumper($self);
  }

When writing your own object class, you can then import C<dump> to be used as a
method, called like so:

  $object->dump;

By assuming that the importing class will provide a certain interface, a
method-exporting module can be used as a simple plugin:

  package Number::Plugin::Upto;
  use Sub::Exporter -setup => {
    into    => 'Number',
    exports => [ qw(upto) ],
    groups  => [ default => [ qw(upto) ] ],
  };

  sub upto {
    my ($self) = @_;
    return 1 .. abs($self->as_integer);
  }

The C<into> line in the configuration says that this plugin will export, by
default, into the Number package, not into the C<use>-ing package.  It can be
exported anyway, though, and will work as long as the destination provides an
C<as_integer> method like the one it expects.  To import it to a different
destination, one can just write:

  use Number::Plugin::Upto { into => 'Quantity' };    

=head2 Mixing-in Complex External Behavior

When exporting methods to be used as methods (see above), one very powerful
option is to export methods that are generated routines that maintain an
enclosed reference to the exporting module.  This allows a user to import a
single method which is implemented in terms of a complete, well-structured
package.

Here is a very small example:

  package Data::Analyzer;

  use Sub::Exporter -setup => {
    exports => [ analyze => \'_generate_analyzer' ],
  };

  sub _generate_analyzer {
    my ($mixin, $name, $arg, $col) = @_;

    return sub {
      my ($self) = @_;

      my $values = [ $self->values ];

      my $analyzer = $mixin->new($values);
      $analyzer->perform_analysis;
      $analyzer->aggregate_results;

      return $analyzer->summary;
    };
  }

If imported by any package providing a C<values> method, this plugin will
provide a single C<analyze> method that acts as a simple interface to a more
complex set of behaviors.

Even more importantly, because the C<$mixin> value will be the invocant on
which the C<import> was actually called, one can subclass C<Data::Analyzer> and
replace only individual pieces of the complex behavior, making it easy to write
complex, subclassable toolkits with simple single points of entry for external
interfaces.

=head2 Exporting Constants

While Sub::Exporter isn't in the constant-exporting business, it's easy to
export constants by using one of its sister modules, Package::Generator.

  package Important::Constants;
 
  use Sub::Exporter -setup => {
    collectors => [ constants => \'_set_constants' ],
  };
 
  sub _set_constants {
    my ($class, $value, $data) = @_;
 
    Package::Generator->assign_symbols(
      $data->{into},
      [
        MEANING_OF_LIFE => \42,
        ONE_TRUE_BASE   => \13,
        FACTORS         => [ 6, 9 ],
      ],
    );

    return 1;
  }

Then, someone can write:

  use Important::Constants 'constants';
  
  print "The factors @FACTORS produce $MEANING_OF_LIFE in $ONE_TRUE_BASE.";

(The constants must be exported via a collector, because they are effectively
altering the importing class in a way other than installing subroutines.)

=head2 Altering the Importer's @ISA

It's trivial to make a collector that changes the inheritance of an importing
package:

  use Sub::Exporter -setup => {
    collectors => { -base => \'_make_base' },
  };

  sub _make_base {
    my ($class, $value, $data) = @_;

    my $target = $data->{into};
    push @{"$target\::ISA"}, $class;
  }

Then, the user of your class can write:

  use Some::Class -base;

and become a subclass.  This can be quite useful in building, for example, a
module that helps build plugins.  We may want a few utilities imported, but we
also want to inherit behavior from some base plugin class;

  package Framework::Util;

  use Sub::Exporter -setup => {
    exports    => [ qw(log global_config) ],
    groups     => [ _plugin => [ qw(log global_config) ]
    collectors => { '-plugin' => \'_become_plugin' },
  };

  sub _become_plugin {
    my ($class, $value, $data) = @_;

    my $target = $data->{into};
    push @{"$target\::ISA"}, $class->plugin_base_class;

    push @{ $data->{import_args} }, '-_plugin';
  }

Now, you can write a plugin like this:

  package Framework::Plugin::AirFreshener;
  use Framework::Util -plugin;

=head2 Eating Exporter.pm's Brain

You probably shouldn't actually do this in production.  It's offered more as a
demonstration than a suggestion.

 sub exporter_upgrade {
   my ($pkg) = @_;
   my $new_pkg = "$pkg\::UsingSubExporter";

   return $new_pkg if $new_pkg->isa($pkg);

   Sub::Exporter::setup_exporter({
     as      => 'import',
     into    => $new_pkg,
     exports => [ @{"$pkg\::EXPORT_OK"} ],
     groups  => {
       %{"$pkg\::EXPORT_TAG"},
       default => [ @{"$pkg\::EXPORTS"} ],
     },
   });

   @{"$new_pkg\::ISA"} = $pkg;
   return $new_pkg;
 }

This routine, given the name of an existing package configured to use
Exporter.pm, returns the name of a new package with a Sub::Exporter-powered
C<import> routine.  This lets you import C<Toolkit::exported_sub> into the
current package with the name C<foo> by writing:

  BEGIN {
    require Toolkit;
    exporter_upgrade('Toolkit')->import(exported_sub => { -as => 'foo' })
  }

If you're feeling particularly naughty, this routine could have been declared
in the UNIVERSAL package, meaning you could write:

  BEGIN {
    require Toolkit;
    Toolkit->exporter_upgrade->import(exported_sub => { -as => 'foo' })
  }

The new package will have all the same exporter configuration as the original,
but will support export and group renaming, including exporting into scalar
references.  Further, since Sub::Exporter uses C<can> to find the routine being
exported, the new package may be subclassed and some of its exports replaced.

=head1 AUTHOR

Ricardo Signes <cpan@semiotic.systems>

=head1 COPYRIGHT AND LICENSE

This software is copyright (c) 2007 by Ricardo Signes.

This is free software; you can redistribute it and/or modify it under
the same terms as the Perl 5 programming language system itself.

=cut
